1. Field of the Invention
This invention relates to a semiconductor device having a composite substrate formed by fixing two semiconductor substrates in close contact with each other, and more particularly to a high speed switching semiconductor device in which it is necessary to rapidly extinguish accumulated charges.
2. Description of the Related Art
Various types of semiconductor devices require a high speed switching operation. For example, an insulated gate bipolar transistor (which is often called simply an IGBT or conductivity modulation type MOS FET and is hereinafter referred to as IGBT) is well known in the art as a high speed switching semiconductor device. An IGBT is disclosed in, for example, U.S. Pat. No. 4,700,466 and EP-0192229, and is constructed by sequentially forming an N.sup.+ -type region of high impurity concentration and an H.sup.- -type region of low impurity concentration on a P.sup.+ -type semiconductor substrate of high impurity concentration by means of epitaxial growth, and then forming a vertical type double diffusion MOS FET (which is hereinafter referred to as a VD MOS FET) in the N.sup.- -type region. In other words, the above IGBT is obtained by additionally forming the P.sup.+ -type region in the N.sup.+ -type drain region of the VD MOS FET.
When the above IGBT is in the ON state, majority carriers (electrons) diffuse from the source region into the drain region, and minority carriers (holes) move from the P+-type region into the drain region. As a result, a large amount of excessive minority carriers remain in the drain region in comparison with a VD MOS FET in the same (i.e. ON) state, and for this reason, the forward voltage (Vf) of the IGBT can be set to be small even when a large current passes therethrough, the IGBT has a high withstanding voltage, as in the case of the VD MOS FET, and can be rapidly turned off through control of the gate voltage.
On the other hand, the turn-off characteristic of the IGBT may be degraded, on account of the presence of the minority carriers accumulated in the drain region, and in order to solve this problem, a method whose purpose is to shorten the lifetime of the minority carriers in the drain region has been employed in the prior art. More precisely, a deep level acting as the center of recombination of the carriers is formed over the entire area of the substrate by applying an electron beam to the substrate or diffusing a heavy metal such as Au, Pt into the substrate. In general, the lifetime controlling method may cause the lifetime of carriers to be shortened and the operation speed of the semiconductor device to be enhanced; however, it does have the disadvantage that the leak current flowing in the semiconductor device set in the forward blocking condition is increased, as is also the forward voltage or ON-voltage (Vf).
As described above, the IGBT is advantageous over the VD MOS FET in that the ON-voltage can be kept low even when a large current is caused to flow, but has the disadvantage that the turn-off characteristic is degraded. Using the prior art technique to solve this problem, results an increase in the leak current as well as in the ON-voltage (Vf).